Preheat oven to 350 degrees F and line a large, rimmed baking sheet or jelly roll pan with parchment paper.
Heat oil in a large skillet set over medium heat. Add your finely diced onions and cook, stirring occasionally, until soft and translucent. Season with salt and add the minced garlic. Cook the garlic and onions for approximately 1 minute before adding the finely diced mushrooms (you may dice them by hand or use a food processor), fresh herbs, and diced apple. Mix well to combine and continue to cook until mushrooms drop their water and most of the liquid boils away (6-8 minutes).
Add the Worcestershire sauce, tomato paste, and chicken stock (or water) to the skillet. Mix well to combine thoroughly. Remove from heat and set aside.
Meanwhile, transfer the ground turkey, breadcrumbs, eggs, salt, and pepper to a large mixing bowl. Use your hands to thoroughly mix and mash the ingredients together until completely combined.
Once the onion and mushroom mixture comes down to room temperature, transfer to the bowl with the ground turkey. Thoroughly mix to combine and set aside.
Whisk together the glaze. Add the ketchup, Dijon mustard, brown sugar, and red wine vinegar to a small mixing bowl. Whisk well to combine. Set aside.
Transfer your meat mixture to the prepared baking sheet lined with parchment paper. Use your hands to shape the meat into a large loaf-shape (you may also add your meat mixture to two greased loaf pans if preferred).
Use a pastry brush to brush the surface of your loaf with half of your pre-made glaze. Reserve the remaining glaze for later.
Bake, tented with foil, for 45 minutes. After 45 minutes, increase the oven temperature to 400 F and remove the foil. Brush the surface with the remaining glaze and continue to bake for 15-20 minutes, or until internal temperature registers 160 degrees F.
Remove from the oven and allow to rest for 5 minutes before slicing.